I made myself a standing treadmill desk for my faculty job, and generally liked it. I got a normal treadmill from Craigslist and a desk from IKEA. I tended to walk a good deal when I had typing to do and the work did not require a full mental effort. I could never get writing to go easily while walking, and there was something about the walking process on the treadmill that required some of my CPU cycles (in contrast to regular walking outside, which does not). So: marking papers using MS Word - yes, marking papers with pen - no, thinking hard about a research problem - no.
I tended to walk about 1-4 miles a day, depending on the type of work I had to do, and how much of it was in the office vs. lab vs. lecture room.
The main drawback with a cheap, regular (not designed for office use), used treadmill was the noise. It was pretty loud. I put some rubber mats under the base to deaden the noise a bit, which was sufficient for my downstairs neighbor, but it was still too loud for me next-door neighbor. That would be the main driver to get a "real" office walking treadmill, which are quieter, and are optimized for walking, not running speeds.